Summary Script

I asked this question earlier but gave an incorrect description of my situation. I need some help setting up the summary tool for “Field 2”. Please see below, I want “Field 2” to do the following. Can someone help me with the correct settings?

It's been my experience that GetNthRecord in such a "chain" of unstored calculations can exhaust system resources and starts returning ? results after your found set exceeds a certain size. Might be better to use GetnthRecord to look up the previous value, copying it into a number field. This has it's own set of drawbacks so your mileage may vary.